The visual color impression of offset-printed products is produced by means of an interaction of subtractive and additive color mixing. In a half-tone print the individual points of each printing ink are printed in various sizes, both next to each other and more or less overlapping each other. Each ink point has a finite thickness and the effect corresponds to a filter lying on the white printed material. The color direction of the combined print is determined both by the layer thickness of the applied printing ink and by the geometric area coverage of the half-tone points. By means of varying the adjustment of the ink feed elements in the individual printing units, the color locus of a printed image point can thus be varied. Generally in color printing, three chromatic colors cyan, magenta, and yellow and a fourth printing ink, black, are printed.
In the printing industry it has been common to use simultaneously printed special measuring fields on the printed product, such as test strips, for the purpose of detecting the ink application on a printed product. The simultaneously printed special measuring fields are photoelectrically detected, and a measure for the applied quantity of ink is derived therefrom. This method is mostly carried out by means of densitometers, as there exists a relatively simple relationship between ink density value and layer thickness of the ink. This ink feed control method has several disadvantages. For example, the densitometric determination of the ink feed permits no numeric statements with regard to the visual color perception. Another disadvantage of monitoring the ink feed on simultaneously printed special measuring elements is that space on the printed material is used for these measuring fields. Furthermore, since color monitoring is done only on the special measuring fields, the ink application is controlled only to achieve the desired color impression of these measuring fields. The color impression of the actual subject is correspondingly only varied indirectly, and correct color impression of the special measuring fields does not guarantee correct color impression of the printed product.
U.S. Pat. No. 5,182,721 discloses a method for the control of ink application in a printing machine, in which method test regions are measured calorimetrically on the printed sheet. Color loci are determined from calorimetric measurements with reference to a selected color coordinate system. Color distances between the actual values of the printed copy and the desired values of the original are formed and the control data are to be determined from these color distances. This method uses a dependence of the color locus coordinates, to be determined empirically, as a function of an alteration of the layer thickness of the printed ink.
This method works calorimetrically, and the determined color loci and color distances permit conclusions about the visual color impression. However, because of the described method of calculating setting commands for the ink feed via the partial derivatives of the color locus coordinates with respect to the color densities of the relevant printing inks, this principle appears to work only in the case of simultaneously printed special measuring fields. This document alone does not disclose how setting commands for the ink feed can be obtained by means of measurements directly on the printed product.
German patent document DD 227 094 A1 discloses a method for the calorimetric evaluation of printed products. By means of measuring devices in the machine, the color locus coordinates of specific test regions are determined and, by means of the Neugebauer relationship, degrees of area coverage of the printed inks are determined therefrom. If the color black is also printed in addition to the chromatic colors,, then a second calorimetric measuring device for this color is necessary, which can be disadvantageous.
U.S. Pat. No. 4,649,502 discloses a method for assessing the print quality as well as for controlling the ink feed. The reflectance is measured in four spectral regions on image elements in the subject using one or more measuring heads. According to the patent, the color reflectance values for the chromatic colors as well as a spectral reflectance in the infrared region for the black printing ink are determined. Area coverage values are determined by means of the Neugebauer equations. This is carried out at the same image points of the copies printed on the machine as on a desired original. Setting commands for the ink feed of the printing machine can be derived from the comparison of desired and actual surface coverages of the printing inks.
From the prior art it is thus known to determine the geometric area coverages of test regions either by means of color reflectance values or color coordinates of the test regions, and to determine therefrom control variables for the ink feed. As is known, the Neugebauer relationship when extended to four colors describes the theoretical relationship between the color locus of a four-color combined print and the degrees of geometric area coverage of the individual colors and their combined prints. In this case the standard color values for the individual colors, the combinations of the combined prints, and the paper-white are determined on full-surface-printed samples. Since light scattering and light gathering are not taken into account, the geometric area coverages determined in this way and the ink feed change determined from a desired v. actual comparison can only deliver inaccurate results. As is known, in a half-tone structure it is the optically effective surface coverage which is decisive of the color impression, not the geometrical surface coverage.